1. Field of Invention
This invention relates to the field of art pertaining to test fixtures for board test systems and other mechanical interfaces for electrically interconnecting circuit cards having electronic components and the like to electrical switching systems.
2. Description of the Prior Art
A board test system consists of numerous electronic sources and detectors which are connected through an electric switch, or scanner, to a plurality of contact points referred to as scanner pins. A test fixture provides a mechanical and an electrical interface between thesee scanner pins and the electronic components located on a circuit card to be tested. Electronic signals are employed by the board test system to determine whether each electronic component on the circuit card is operating properly. Since these signals must pass through the test fixture both on their way to and from the electronic component, the test fixture must not degrade the quality of these signals in order to insure that the electronic component is correctly diagnosed as operating properly or improperly.
Prior art vacuum actuated test fixtures are typically constructed from two plates. The first plate, or probe plate, is a thick plate made from an insulator having holes corresponding to the locations of the electronic components on the circuit card. The probing means are mounted in these holes. Typically the probing means comprises a spring loaded probe and a probe socket. A second plate, or top plate, having holes corresponding to the location of the probing means, is mounted on alignment pins and held above the probe plate by preloaded fixture springs. A seal is then placed around the perimeter of the plates to form a vacuum chamber. When air is removed from the vacuum chamber, the top plate is drawn toward the probe plate causing each probing means to pass through its corresponding hole in the top plate and strike an electronic component on the circuit card. The circuit card is positioned relative to the top plate by two or more tooling pins and held in place by the vacuum. Some prior art fixtures have replaced the top plate by a flexible membrane.
In order to ensure a high quality signal path to and from the test system, the probing means must make a firm electrical and mechanical contact with the component. Two essential factors for making a good electrical and mechanical contact are, first, proper alignment between the probing means and the electronic components and, second, a uniform force by the probing means against the component. Although these factors are easily achieved for a few probing means in a test fixture, it is very difficult to achieve a good electrical and mechanical contact for each probing means over the entire test fixture. There are two primary reasons for these difficulties in the prior art. First, the force from the spring loaded probing means must be carefully balanced against the force from the fixture springs located between the top and probe plates. If the fixture springs are too weak, the circuit card will pop off during the test and if they are too strong, the circuit card will not be drawn down properly making a poor connection. If the spring loaded probing means is too strong, the circuit card will pop off during the test and if they are too weak, the circuit card will not be probed properly again making a poor connection. A poor connection between the probing means and the electronic component somewhere in the test fixture usually results from having to make this tradeoff. Second, since the alignment with the circuit card is provided by tooling pins in the top plate, a precise registration must also be maintained between the top plate and probe plate making the fixture more expensive to manufacture and less accurate.
Another problem with the prior art vacuum actuated fixtures is difficult maintenance. With the prior art vacuum test fixture, dirt and solder particles are often sucked into the fixture requiring the fixture to be opened. The seals around the perimeter of the plates also wear out and need replacement. However, opening the fixture is difficult, since any misalignment during disassembly or reassembly could damage the probing means and the springs must be carefully removed during disassembly and replaced during reassembly. Therefore, fixture repair is a very time consuming process.
Yet another problem with the fixture springs of the prior art test fixture is the large quantity of vacuum required to actuate the fixture and the noise made while the fixture is actuated. The fixture springs also cause the circuit card to warp when the fixture is actuated which complicates maintaining proper registration of the probing means and selecting of the proper spring force for the probing means. In addition, the circuit card may be permanently damaged by the force. In order to prevent the top and probe plate from being warped, thicker plates are used adding additional weight of the test fixture. Even with thicker plates, some fixtures require the use of stop supports between the top and probe plates. In order to prevent the circuit card from being warped, supports are placed between the circuit card and the top plate.
A few single plate fixtures have been developed. These fixtures comprise a probe plate which is almost identical to the probe plate of the two plate fixture. A thick layer of foam is placed directly on top of the probe plate. The foam is removed from the probe plate directly under the circuit card except for the approximately one-half inch around the perimeter of the circuit card which serves as the vacuum seal. The primary disadvantage of such a fixture is that the probing means are always exposed when the fixture is not actuated. The exposed probing means may be easily damaged or cause injury to an operator. Also these test fixtures generally make a poorer contact than the two plate fixture because there is limited travel of the circuit card against the foam seal. Another disadvantage of this fixture is that the circuit card is not supported while the fixture is being actuated making it more susceptible to damage by warping. Such damage may include breaking electronic components, solder joints or both.
What is required is a test fixture which is capable of making a repeatable and reliable high quality connection between the electronic components on the circuit card and the board test system.